A full range diesel pool in refinery comprises of various streams from primary units such as crude & vacuum distillation units and secondary conversion units like FCC, visbreaker, resid FCC, delayed coker, etc. These various streams constitute varying concentrations of various organo-sulfur compounds and varying concentrations of paraffinic, naphthenic and aromatic compounds. The diesel specifications for sulfur and cetane number are the two major properties which are targeted to achieve by hydrotreating family of reactions. The hydrotreating reactions include hydrogenation, hydrogenolysis, isomerization associated with some undesired thermal and catalytic cracking leading to formation of coke and lighter hydrocarbons.
The sulfur compound species found in the diesel pool can be broadly categorized into two types namely: ‘easy sulfur’ type species and ‘difficult or refractory sulfur’ type species. The ‘easy sulfur’ species undergoes desulfurization in hydrotreating by hydrogenolysis reaction mechanism. The reaction is much faster and hence diesel streams constituting the easy sulfur species require lesser amount of catalyst volume per unit volume of feed per hour (i.e. less reaction time), lesser temperatures, and pressures. Also, the diesel streams constituting ‘easy sulfur’ are composed of higher paraffins and naphthenic compounds and lesser aromatic compounds. Hence, for cetane improvement of these streams one require again lesser amount of catalyst volume per unit volume of feed per hour (i.e. less reaction time), lesser temperatures, and pressures. On the other hand, the ‘difficult or refractory sulfur’ species needs to be first hydrogenated and then hydrogenolysis reaction. This reaction is slower and hence the diesel streams constituting the difficult or refractory sulfur species require higher amount of catalyst volume per unit volume of feed per hour (i.e. more reaction times), higher temperatures, and pressures. Also, the diesel streams constituting ‘difficult or refractory sulfur’ are composed of lesser paraffins and naphthenic compounds and higher aromatic compounds. Hence, for cetane improvement of these streams one require again higher amount of catalyst volume per unit volume of feed per hour (i.e. more reaction time), higher temperatures, and pressures.
It is known in the art that ‘difficult or refractory sulfur’ species and higher aromatics are relatively more concentrated in higher boiling part of full range straight run diesel streams. Further, the ‘difficult or refractory sulfur’ species are also present in the diesel range streams obtained from the secondary conversion units like FCC and delayed Coker units. The concentrations of aromatics are also high in these streams compared to the straight run diesel streams. Further, it is also known in the art that the diesel streams comprising higher concentrations of ‘difficult or refractory sulfur’ species contains not only higher concentrations of mono-aromatics but also higher concentrations of aromatics having two or more rings. These compounds require higher catalyst volumes i.e. more reaction time, and higher pressures and temperatures to ‘treat’ them effectively. The term ‘treat’ means removal for sulfur from sulfur species and deep saturation of aromatics.
U.S. Pat. Nos. 6,126,814, 6,013,598, and 5,985,136 discloses hydrodesulfurization processes, wherein the diesel with high sulfur content goes through two consecutive stages of hydrogen treatment: the first stage removes smaller sulfur compound molecules and thereafter the second stage removes larger molecules. In a typical two stage hydrodesulfurization process, the first stage operates at a temperature of about 300° C. and a pressure of about 44 barg. The high temperature and pressure is necessary to reduce the wetting barrier between solid, diesel, and hydrogen. The second stage operates at a temperature of about 400° C. and a pressure of about 58 barg. The higher temperature in the second stage is required to mitigate the higher resistance to mass transfer of the more stearically hindered sulfur compounds such as benzothiophenes, dibenzothiophenes, etc.
However, the hydrodesulfurization process not only reduces the amount of sulfur in the fuel, but also saturates olefins and reduces the amount of other heteroatom-containing compounds, including nitrogen-containing and oxygen-containing compounds in the fuel. Further the process also saturates the aromatic amount in the middle distillate, thereby improving the cetane number (an very important parameter) of the Diesel. It is widely known that the hydrodesulfurization reaction also involves some undesired thermal and catalytic cracking leading to formation of coke and lighter hydrocarbons and thereby generating unwanted dry gas (methane and ethane) and wild naphtha. These unwanted reactions in the hydrodesulfurization process can be minimized by optimizing the contact time of feed with catalyst. The optimization of contact time is also very vital to achieve ultra-low sulfur levels (below 10 ppmw) in the fuel. The reaction products formed due to hydrodesulfurization and other associated reaction also contains H2S and NH3 having inhibition effect on the hydrodesulfurization reaction itself. However, the presence of optimum quantity of H2S in the reactor system is also very important for maintaining catalyst in active form. Therefore, appropriate staging effect is required to maintain only the optimum quantity of H2S in the reactor system.
Hence, it can be seen from the aforementioned that there remains a need in the art for an improved process for removing sulfur compounds from petroleum-based fuel that overcomes the deficiencies of the prior art.
The present invention provides a process configuration for deep desulfurization and deep hydrotreating of diesel range hydrocarbons to obtain diesel products by optimizing the contact time of feed with catalyst system and providing efficient staging effect. The efficient staging effect means maintaining optimum amount of H2S in the reactor, so as to reduce the inhibition effect due to H2S without hampering the catalyst activity.